1. Field of the Invention
The present invention relates to a fuel cell system employed in a fuel cell-powered vehicle or the like, and relates to an operation method therefor.
Priority is claimed on Japanese Patent Application No. 2002-332183, filed Nov. 15, 2002, the content of which is incorporated herein by reference.
2. Description of Related Art
As a type of fuel cell to be installed in a fuel cell powered vehicle, a fuel cell is known which includes a fuel cell unit having an anode and a cathode that together sandwich a solid polymer electrolyte membrane therebetween, and in which a fuel gas (e.g., hydrogen gas) is supplied to the anode, and an oxidizing gas (e.g., oxygen or air) is supplied to the cathode so that electrochemical energy produced in an oxidation-reduction reaction of these reaction gases is directly output as electrical energy.
Because an anode off-gas (hereinafter referred to as a hydrogen off-gas) discharged from the anode side of the fuel cell includes unreacted hydrogen, fuel economy is reduced when such a hydrogen off-gas is expelled as it is. Accordingly, in order to improve fuel economy, a type of fuel cell system has been proposed in which the hydrogen off-gas is actively circulated, is mixed with a fresh hydrogen gas, and is resupplied to the fuel cell.
For example, Japanese Unexamined Patent Application, First Publication No. S58-30075 discloses a fuel cell system in which a hydrogen off-gas is circulated using an ejector (an ejector pump) so that the hydrogen off-gas is resupplied to the fuel cell.
Moreover, Japanese Unexamined Patent Application, First Publication No. H07-240220 discloses a fuel cell system in which a hydrogen gas is recycled by providing a pump or compressor in a circulation path for the hydrogen gas and by operating the pump or compressor.
However, in the above-mentioned fuel cell system provided with the ejector, a problem is encountered upon starting the fuel cell system. More specifically, in order to circulate the hydrogen gas using the ejector, flow of the hydrogen gas must be produced in the circulation path; therefore, when there is no hydrogen gas flow in the circulation path at starting or the like of the fuel cell system, a forced flow of the hydrogen gas must be produced in the circulation path. When hydrogen is purged from the hydrogen circulation path in order to produce the forced flow, a problem is encountered in that fuel economy is reduced because unreacted hydrogen gas is discharged from the circulation path.
On the other hand, in the above-mentioned fuel cell system provided with the pump or compressor, because the pump or compressor must be continuously operated during circulation of the hydrogen gas, electrical power for operating the pump or compressor is continuously consumed, and thus, a problem is encountered in that fuel economy is reduced by an amount corresponding to the consumed electrical power.